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From what you've mentioned there, I can't understand why you'd want to use a mono amplifier for vocals, 8 inch driver for vocals ! you need a front soundstage and accurate STEREO imaging for vocals - not mono.

Capacitors are not required and serve no usefulness as the amplifiers have quality internal power supplies. Ensure your power wiring, ground and fusing is all well installed, perhaps an audio grade battery as well.

The amplifiers you've got are quite powerful in RMS output. The C5's are the speakers which should be in your doors and perhaps in a custom door build, one of the 4 channel amps to power those and the rear speakers off the remaining 2 channels. The 2nd 4 channel amplifier isn't really needed I'd say. Use an 8 or 10 or 12 inch subwoofer driven from the mono amplifier.

Factor in costs like fabrication of door builds, subwoofer enclosure to fire through ski hatch (make one if you don't have one) , fitting, wiring, power protection etc etc.

I used to have many E30's in the 90's and so did my younger brother. To date, I haven't seen anyone else do what we did in one car, which was to squeeze TWO 18 inch subwoofers into the trunk/boot

I also suggest you take a look at some dedicated car audio forums for more advice